Thirty food and clinical isolates of Listeria were compared quantitatively in regard to lethality in immunocompromised mice, hemolytic activity for sheep erythrocytes, invasiveness towards Hep-2 epithelial cells, and cytotoxicity to CHO cells. All Listeria monocytogenes isolates were hemolytic, invasive, weakly cytotoxic, and lethal to immunocompromised mice. Listeria ivanovii isolates expressed the first three properties but were non-virulent. There was little quantitative correlation among the virulence markers, suggesting that there may be additional virulence related factors that may influence the pathogenicity of L. monocytogenes isolates. No systematic differences between the clinical and food isolates were apparent. Electron and light microscopy of infected Hep-2 cells revealed L. monocytogenes and L. ivanovii encapsulated within cell processes containing an actin matrix.